Heat transport of the kagome Heisenberg quantum spin liquid candidate YCu3(OH)6.5Br2.5 : Localized magnetic excitations and a putative spin gap

The spin-1/2 kagome Heisenberg antiferromagnet is generally accepted as one of the most promising two-dimensional models to realize a quantum spin liquid state. Previous experimental efforts were almost exclusively on only one archetypal material, the herbertsmithite ${\mathrm{ZnCu}}₃{(\mathrm{OH})}₆{\mathrm{Cl}}₂$, which unfortunately suffers from the notorious orphan spin problem caused by magnetic disorders. Here, we turn to ${\mathrm{YCu}}₃{(\mathrm{OH})}_6.5{\mathrm{Br}}_2.5$, recently recognized as another host of a globally undistorted kagome ${\mathrm{Cu}}²⁺$ lattice free from orphan spins, and thus a more feasible system for studying the intrinsic kagome quantum spin liquid physics. Our high-resolution low-temperature thermal conductivity measurements yield a vanishingly small residual linear term of $\ensuremath{\kappa}/T$ ($T\ensuremath{\rightarrow}0$), and thus clearly rule out itinerant gapless fermionic excitations. An unusual scattering of phonons grows exponentially with temperature, suggesting thermally activated phonon-spin scattering and hence a gapped magnetic excitation, consistent with a ${\mathbb{Z}}₂$ quantum spin liquid ground state. Additionally, an analysis of the magnetic field impact on the thermal conductivity reveals a field closing of the spin gap, while the excitations remain localized.